The microsporidia are "emerging" human and veterinary pathogens that cause disease in both immunocompromized and immunocompetent hosts. Our efforts are directed at understanding the polar tube, an organelle that is critical for cell invasion. The polar tube serves as a unique vehicle for transmission of infection by discharging from the microsporidian spore, piercing an adjacent host cell, and inoculating the sporoplasm directly into that cell. While described as a unique microsporidian structure over 100 years ago, the biochemical components of this structure and the mechanism of its formation during invasion remain to be definitively determined. Further study of the composition, formation and function of this organelle during germination and invasion should provide a basis for the development of new strategies for control of these important parasitic protists. We have identified modifications and motifs that occur in the major polar tube protein (PTP1) and plan to complete the characterization of the proteins (PTPs) involved in the formation and construction of the polar tube. The Encephalitozoon cuniculi genome will be utilized to undertake a proteomics approach to complete the characterization of these PTPs. The molecular interaction(s) of PTP1 with itself and with other PTPs will be evaluated. This molecular characterization will be complemented by ultrastructural studies investigating the development and location of PTPs during the morphogenesis and eversion of the polar tube. Observations from our -laboratory suggest that the polar tube may be extracellular in the microsporidian spore. Finally, the process of germination will be investigated by characterizing at the molecular and morphologic level a putative microsporidian aquaporin (AQP) that we have identified. Understanding germination is important in characterizing how the invasion organelle (i.e. polar tube) functions. It has been hypothesized that an increase in osmotic pressure and influx of water through an AQP is the mechanism by which germination and the subsequent eversion of the polar tube occurs. We believe that the information gained by delineating, in detail, the function and components of the polar tube and mechanism of germination could eventually lead to novel therapeutic interventions that would limit or interdict the transmission of these emerging pathogens.